128 J. H. Pratt — Northujyite^ Pirssonite^ 



prismatic crystals are often only well terminated at tliat end 

 where the e faces occur. 



The following table includes a list of the measured and cal- 

 culated angles. 



As the reflections were not always very perfect, the 

 extremes of two or more independent measurements are given. 







Measured. 



Mean. 



Calculated. 



p^p" 



111 Alll 



62°57' 



-63° 3' 



=^63° 0' 





m A m'" 



llOAllO 



59° 



-59° 4'30" 



*59° 2' 





p Ah 



lllAOlO 



74°51'30' 



'-75° 7' 



75° 1'33" 



tb o 



p A m 



lllAllO 



58°32' 



-58°47' 



58°38'15" 



58°30' 



pAp' 



lllAlll 



53°57' 



-54° b' 



53°59'36" 



54° 6' 



pAp'^' 



niAiIi 



29°54' 



-29°59'30" 



29°57'30" 



29°50' 



b Ae 



010 A 131 



51°14' 



-51°43' 



51°26'50" 



51°22' 



eAe'" 



131A131 



76°56' 



-77°12' 



77° 4' 



77°16' 



XAX" 



311 A311 



18°12'48' 



/ 



18°12'48"" 



18°10'30" 



XAm 



311A110 



36°44' 



-36°45' 



36°44'30" 



36°15'20'' 



XAm'" 



311 Alio 



49°26' 



— 49°23' 



49°24'30" 



49°24' 



Physical properties. — The crystals are extremely brittle, 

 breaking with a conchoidal fracture, but with no apparent 

 cleavage. The luster is vitreous. They vary from colorless to 

 white, but are often darkened by impurities. The hardness is 

 between 3 and 3*5. The specific gravity, taken by means of 

 the heavy solution, was found to be ii-352. 



The crystals exhibit the phenomena of pyroelectricity in a 

 marked degree. While cooling, after being gently heated, the 

 extremity upon which the acute pyramid e, 131, is developed, 

 became negatively electrified. This was shown by dusting 

 with a mixture of red oxide of lead and sulphur, as recom- 

 mended by Kundt."^ 



Optical properties . — The plane of the optic axes is the base, 

 and the axis h is the acute bisectrix. The optical orientation is 

 a = a, b = <3 and c = &. The double refraction is positive and 

 strong. The dispersion is slight p <i v. 



For the determination of the indices of re^i'action the 

 method of total reflection was employed, making use of a 

 crystal upon which a large pinacoid face, h, 010, was developed. 

 The plate was measured in a monobromnaphthalene, whose 

 index of refraction for yellow, Na, was found to be 1*6588 at 

 23° C. The values obtained were : 



For yellow, Na, a = 1*5043 

 /3= 1*5095 

 y= 1*5751 



By means of the three indices of refraction the value of 

 Ya.y was calculated and found to be 16° 24'. 



* Ann. d. Phys. u. Chem , xx, p. 592, 1883. 



